To increase the reaction rate in fuel cells and enhance the energy conversion efficiency, a layer containing a catalyst (hereinafter, also the fuel cell catalyst layer) is conventionally provided on the surface of a cathode (an air electrode) or an anode (a fuel electrode) of a fuel cell.
Noble metals are generally used as the catalysts. Of the noble metals, platinum that is stable at high potential and has high catalytic activity is most frequently used. However, since platinum is expensive and exists in a limited amount, alternative catalysts have been desired.
Further, the noble metals used on a cathode surface are often dissolved in an acidic atmosphere and are not suited in applications requiring long-term durability. Accordingly, it has been strongly demanded that catalysts are developed which are not corroded in an acidic atmosphere and have excellent durability and high oxygen reducing ability.
Materials containing nonmetals such as carbon, nitrogen and boron capture attention as alternative catalysts to platinum. The materials containing these nonmetals are inexpensive compared to noble metals such as platinum and are abundant. Processes for the production of catalysts with such materials have been studied in universities and research institutes.
Nonpatent Document 1 reports that zirconium-based ZrOxN compounds show oxygen reducing ability.
Patent Document 1 discloses, as platinum-alternative materials, oxygen-reducing electrode materials containing a nitride of one or more elements selected from Groups 4, 5 and 14 in the long periodic table.
However, the materials containing these nonmetals do not provide sufficient oxygen reducing ability for practical use as catalysts.
Patent Document 2 discloses an oxycarbonitride obtained by mixing a carbide, an oxide and a nitride and heating the mixture in vacuum or an inert or non-oxidative atmosphere at 500 to 1500° C.
However, the oxycarbonitride disclosed in Patent Document 2 is a thin-film magnetic head ceramic substrate material, and the use of the oxycarbonitride as catalyst is not considered therein.
Meanwhile, platinum is useful not only as a fuel cell catalyst as described above but as a catalyst in exhaust gas treatment or organic synthesis. However, the expensiveness and the limited amount of platinum have created a need of alternative catalysts in these applications too.
Patent Document 3 discloses a process for producing catalyst materials including a step of forming an amorphous material of a metal oxide and a step of crystallizing the metal oxide by heating. Patent Document 3 describes that the metal oxides are used for the oxidation of harmful contaminants in the absence of UV illumination. The use of the metal oxides as fuel cell catalyst materials is not considered.
Patent Document 1: JP-A-2007-31781
Patent Document 2: JP-A-2003-342058
Patent Document 3: JP-A-2008-504957
Nonpatent Document 1: S. Doi, A. Ishihara, S. Mitsushima, N. Kamiya, and K. Ota, Journal of The Electrochemical Society, 154 (3) B362-B369 (2007)